18. Growth responses of green plants

18 Growth responses of green plants 18.1 Do plants respond to external stimuli? 18.2 What are tropisms? 18.3 The growth response of shoots to light 18.4 The growth response of roots and shoots to gravity 18.5 The growth response of roots to water 18.6 What are the substances for controlling growth movements in plants? 18.7 More about auxins 18.8 The role of auxins in phototropic and geotropic responses Mind Map

Do Plants Respond to External Stimuli?

? YES! Plants Will it happen Stimulus Response 18.1 Do plants respond to external stimuli? ? Will it happen Stimulus YES! Plants Response

18.2 What are tropisms? What are Tropism?

growth 18.2 What are tropisms? Tropism stimulus growth Tropism directional growth movement made by a plant Response in response to a unilateral stimulus plant

18.2 What are tropisms? Direction of growth towards stimulus away from stimuli negative tropism positive tropism stimulus stimulus plant plant

18.2 What are tropisms? light water gravity Phototropism Geotropism Hydrotropism H2O water gravity

The Growth response of Shoots to Light

18.3 The growth response of shoots to light Phototropism light shoot root

Positive phototropism Negative phototropism 18.3 The growth response of shoots to light Phototropism light Leaves can gain maximum amount of light for photosynthesis Importance: Positive phototropism shoot Negative phototropism root

18.3 The growth response of shoots to light Clinostat

18.3 The growth response of shoots to light Clinostat cancel out the effects of unilateral light & gravity by rotating the disc direction of rotation set up as a control for investigation of growth responses of roots & shoots to unilateral light & gravity

Experiment 18.1 18.3 The growth response of shoots to light To investigate the phototropic response of shoots Select three pots of young seedlings of similar size and label them as pots A, B and C respectively. 2. Put pot A in a light-proof box with a small window on one side on that light reaches the shoot from one direction only. This is unilateral light. unilateral light light-proof box set-up A

Experiment 18.1 18.3 The growth response of shoots to light To investigate the phototropic response of shoots Set up pot B in a similar way except that it is put on a clinostat. Switch on the clinostat. Put pot C in complete darkness. Both pots B and C are controls. Observe the plants after 1 or 2 days. set-up B unilateral light clinostat in complete darkness set-up C light-proof box

Experiment 18.1 18.3 The growth response of shoots to light To investigate the phototropic response of shoots Question 1: What has happened to the shoots of plant A? Explain. Answer: Seedlings in pot A are exposed to unilateral light. The shoots respond by growing towards the light source.

Experiment 18.1 18.3 The growth response of shoots to light To investigate the phototropic response of shoots Question 2: What has happened to the shoots of plant B? Explain. Answer: In pot B, as the effect of unilateral light is cancelled out by the revolution of the clinostat, the shoots do not show any curvature but grow vertically upwards.

Experiment 18.1 18.3 The growth response of shoots to light To investigate the phototropic response of shoots Question 3: What has happened to the shoots of plant C? Explain. Answer: In pot C, in complete darkness, the shoots grow vertically upwards. The plant becomes taller with slender and longer internodes, and carries smaller leaves which are yellow in colour. This condition is called etiolation.

Experiment 18.1 18.3 The growth response of shoots to light To investigate the phototropic response of shoots Question 4: What is the significance of phototropism to plants? Answer: The shoot tends to grow towards the light source. This phototropism ensures that a plant gets maximum light energy for photosynthesis.

The Growth response of Roots & Shoots to Gravity

ground Gravity 18.4 The growth response of roots & shoots to gravity Geotropism ground shoot root Gravity

ground Gravity Negative Geotropism Positive Geotropism 18.4 The growth response of roots & shoots to gravity Geotropism Importance: grow deep into soil to get a firm anchorage Negative Geotropism Positive Geotropism Importance: leaves can receive maximal sunlight for photosynthesis roots can absorb maximum amount of water and mineral salts ground shoot root Gravity

Experiment 18.2 18.4 The growth response of roots & shoots to gravity To study the geotropic response of roots and shoots Add some moist wool in two Petri dishes. 2. Place three bean seedlings on the cotton wool in each Petri dish. Seedling P – placed horizontally Seedling Q – with radicle pointing upwards Seedling R – with radicle pointing downwards

Experiment 18.2 18.4 The growth response of roots & shoots to gravity To study the geotropic response of roots and shoots Cover the lid and place the whole Petri dish A with its edge on a support. Attach Petri dish B onto the cork disc of a clinostat. Set-up A Set-up B support in complete darkness Petri dishes plumule radicle P1 P2 Q1 R1 Q2 R2

Experiment 18.2 18.4 The growth response of roots & shoots to gravity To study the geotropic response of roots and shoots Put both set-ups into a light-proof box. Switch on the clinostat in set-up B. Observe the seedlings after 1 or 2 days. Set-up A Set-up B support in complete darkness Petri dishes plumule radicle P1 P2 Q1 R1 Q2 R2

Experiment 18.2 18.4 The growth response of roots & shoots to gravity To study the geotropic response of roots and shoots Question 1: What has happened to the radicles and plumules in set-up A? Draw a diagram to show the appearance of the seedlings. P1 Q1 R1 Answer: The radicles grow downwards, while the plumules grow upwards.

Experiment 18.2 18.4 The growth response of roots & shoots to gravity To study the geotropic response of roots and shoots Question 2: What has happened to the radicles and plumules in set-up B? Draw a diagram to show the appearance of the seedlings. Answer: All the radicles and plumules grow straight because the effect of gravity is cancelled out by the revolution of the clinostat. P2 Q2 R2

Experiment 18.2 18.4 The growth response of roots & shoots to gravity To study the geotropic response of roots and shoots Question 3: Why is it necessary to leave the apparatus in darkness? Question 4: Do the radicles and plumules show positive geotropism or negative geotropism? Answer: The apparatus is left in darkness to avoid the stimulus of light. Answer: In set-up A, the radicles grow downwards and are positively geotropic, while the plumules grow upwards and are negatively geotropic.

Experiment 18.2 18.4 The growth response of roots & shoots to gravity To study the geotropic response of roots and shoots Question 5: What advantage do the seedlings have by showing such geotropism? Answer: Being positively geotropic, roots grow downwards deep into the soil to provide a firm anchorage underground and absorb maximum water and mineral salts in the soil. Being negatively geotropic, shoots grow upwards to ensure that a plant gets enough light energy for photosynthesis.

The Growth response of Roots to Water

Shoot Root Wet Dry 18.5 The growth response of roots to water Hydrotropism Dry Wet Shoot Root

Positive Hydrotropism 18.5 The growth response of roots to water Hydrotropism Dry Wet No response Importance: Shoot get maximum water as a raw material for photosynthesis Root Positive Hydrotropism get maximum mineral salts to maintain normal growth

Experiment 18.3 18.5 The growth response of roots to water To investigate the relative effects of gravity and water on the growth response of green plants Prepare a 20cm X 20cm gauze square. Bend it to form a shallow tray that is 2cm deep. Place an even 1-cm deep layer of damp vermiculite. Sprinkle about 20 pea seedlings onto the vermiculite. Cover the seedlings with damp vermiculite.

Experiment 18.3 18.5 The growth response of roots to water To investigate the relative effects of gravity and water on the growth response of green plants Set up the apparatus in darkness as shown in the diagram. Observe the results after 2 days. water

Experiment 18.3 18.5 The growth response of roots to water Question: To investigate the relative effects of gravity and water on the growth response of green plants Question: Which response is stronger, hydrotropic or geotropic? Explain. Answer: Some pea seedlings reverse their normal geotropic response and grow upwards towards the damp vermiculite. This shows that hydrotropic response is stronger than geotropic response. water

What are the substances for controlling 18.6 What are the substances for controlling growth movements in plants? What are the substances for controlling Growth Movements in Plants?

18.6 What are the substances for controlling growth movements in plants? coleoptile a protective sheath surrounding the young shoot of the embryo in plants of the grass family first leaf used in the experiments for finding out the factors controlling the growth movements in plants grass seed

Charles Darwin (1880) Experiment Result Conclusion / Explanation 18.6 What are the substances for controlling growth movements in plants? Charles Darwin (1880) Conclusion / Explanation Experiment Result light The coleoptile grew towards the light. coleoptile

Charles Darwin (1880) Experiment Result Conclusion / Explanation 18.6 What are the substances for controlling growth movements in plants? Charles Darwin (1880) Conclusion / Explanation Experiment Result decapitated coleoptile light The shoot tip was sensitive to light.

Charles Darwin (1880) Experiment Result Conclusion / Explanation 18.6 What are the substances for controlling growth movements in plants? Charles Darwin (1880) Conclusion / Explanation Experiment Result opaque cap light This further confirmed that the tip was sensitive to light.

gelatin allows substances to diffuse down 18.6 What are the substances for controlling growth movements in plants? Boysen-Jensen (1913) Conclusion / Explanation Experiment Result light gelatin allows substances to diffuse down A substance produced at the tip diffused through the gelatin down the shaded side. light

it does not allow substances to diffuse down 18.6 What are the substances for controlling growth movements in plants? Boysen-Jensen (1913) Conclusion / Explanation Experiment Result light mica plate Substance causing the bending movement moved down the shaded side of the shoot. it does not allow substances to diffuse down

Paal (1919) Experiment Result Conclusion / Explanation 18.6 What are the substances for controlling growth movements in plants? Paal (1919) Conclusion / Explanation Experiment Result A substance had diffused from the tip & stimulated growth.

Fritz Went (1928) Experiment Result Conclusion / Explanation 18.6 What are the substances for controlling growth movements in plants? Fritz Went (1928) Conclusion / Explanation Experiment Result The substance passed from the coleoptile tip into the agar block and then down into the stump. The stump bent away from the side with a source of that substance. agar block stump

Went named this substance auxin 18.6 What are the substances for controlling growth movements in plants? Fritz Went (1928) Result Experiment Result Went named this substance auxin agar block stump

18.7 More about auxins More about auxin

18.7 More about auxins organic substances plant hormone auxin can regulate growth & other physiological processes auxin auxin auxin auxin act on parts of the plant other than the part that produces them

18.7 More about auxins Site of production shoot tip produces auxins auxins are transported downwards to the root roots tip also produces a small amount of auxins

18.7 More about auxins Site of production auxin exert their effect at 2 3 4 5 6 7 8 auxin exert their effect at shoot tip produces auxins region of elongation 1 2 3 4 5 6 7 8 mark at 2 mm intervals

18.7 More about auxins shoot root Effect of auxins on different parts of a plant Stimulate shoot growth Stimulate root growth No effect on shoot growth Higher conc. Inhibit root growth % growth stimulation Lower conc. shoot root % growth inhibition Auxin concentration / parts per million (p.p.m.)

Geotropic Phototropic The Role of Auxins in & responses 18.8 The role of auxins in phototropic & geotropic responses The Role of Auxins in & Phototropic Geotropic responses

Phototropic response of shoots 18.8 The role of auxins in phototropic & geotropic responses Phototropic response of shoots auxins unilateral light Unilateral light causes auxins to accumulate on the shaded side of the shoot 1

2 3 4 18.8 The role of auxins in phototropic & geotropic responses Higher concentration of auxins stimulates shoot growth 3 unilateral light The shaded side grows faster than the illuminated side The shoot grows & bends towards the light 4

Geotropic responses of shoots and roots 18.8 The role of auxins in phototropic & geotropic responses Geotropic responses of shoots and roots gravity Auxins diffuse down to the lower side due to gravity

18.8 The role of auxins in phototropic & geotropic responses high concentration of auxins inhibits root growth high concentration of auxins stimulates shoot growth lower side of the root grows more slowly lower side of the shoot grows faster than the upper side the root bends downwards the shoot bends upwards

Mind Map Tropism growth movement phototropism geotropism hydrotropism is caused by growth movement includes phototropism geotropism hydrotropism unilateral stimulus auxins nature of stimulus nature of stimulus nature of stimulus which are light water pull of gravity plant hormones produced by result illuminated unilaterally on shoots on shoots and roots positive response by roots shoot tips and root tips auxins diffuse down to lower side auxins move to shaded side importance stimulate growth at so the shoots so the roots roots can absorb more water and mineral salts so the shoots region of elongation in shoots and roots bending toward light source bending upward bending downward roots can penetrate into soil for anchorage and to absorb more water and mineral salts leaves gain maximum amount of light for photosynthesis result result result import-ance positive response by shoots negative response by shoots positive response by roots import-ance

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